All three of the 16-year-old boys survived, report Colin Kane, MD, and colleagues at the University of Texas Southwestern Medical Center. Kane's team treated each of the boys. The teens obtained and smoked the K2 products in separate incidents.

"They certainly did damage to their hearts," Kane, a pediatric cardiologist, tells WebMD. "Hopefully there will be no long-term health implications, just a scare. But time will tell."

In an effort to skirt state and federal laws, K2-type products usually are sold as incense, plant food, or even as toy cleaner. With a nod and a wink, they are marked "not for human consumption." But the herbs are spiked with synthetic cannabinoids -- designer drugs that mimic marijuana.

Although the effects of K2 drugs are similar to those of marijuana, the drugs have little in common with the real herb. Synthetic cannabinoids were initially created for lab use only. They are based on chemicals that never would be approved for human use. As a result, most drug screens do not detect these drugs.

Invoking its emergency authority, the Drug Enforcement Administration (DEA) has made five of these drugs illegal. But new versions pop up all the time. And the old ones are still widely available in products sold on the Internet, in specialty shops, and at convenience stores.

"They did comment it was very easy to get. They had no trouble buying it at gas stations, and they said lots of other kids were doing it," Kane says. "But one of the kids was a high school football player, and he was attracted by the idea it never would show up on drug screens."

K2 Heart Attacks in Teens

Each of the boys suffered chest pains one day to one week after smoking a K2 product. Examination at the emergency department showed that they had suffered the same kind of heart attacks.

It's not yet clear exactly how K2-type drugs cause heart attacks. There are reports in the medical literature of healthy young people having heart attacks after smoking marijuana. These reports are relatively rare, given how widely marijuana is used.

The heart effects of marijuana are likely due to the drug's ability to bind to cannabinoid receptors, switches on brain and body cells that trigger different functions. Synthetic cannabinoids bind more strongly to these switches than do the cannabinoids in marijuana. This results in stronger potency and more intense effects.

"We don't know everything about these drugs, but there are certainly health risks," Kane says. "This should not be seen as a safe alternative to marijuana."

Kane and colleagues report the three cases in the December issue of Pediatrics.